At present, carburetor choke valves are primarily divided into two kinds—automatic and manual. Under natural conditions, a carburetor choke valve in the automatic mode is fully closed, and after engine startup the choke block is driven to rapidly rotate by the forced air generated by the rotation of flywheel blades and the throttle control lever is driven by the choke block, causing the rotation of the carburetor choke valve shaft to open the choke valve. However, due to the high rotational speed of the flywheel and the rapid increase of the driving force of the forced air generated after engine startup, the choke valve is immediately opened to the maximum, thus resulting in a lower concentration of mixed gas entering the carburetor, and starting the engine smoothly becomes difficult and a hunting phenomenon emerges. In addition to that, as a mixed gas concentration on the low side is required when the engine is warm started and a conventional choke valve is still fully closed when being manually started a mixed gas concentration on the high side will make starting the engine difficult. In the manual mode, manual control of the extent of opening of the choke valve is required and this causes operation inconvenience.